Answer: In your winter hardiness zone 4, the only reliably hardy hydrangeas would be Hydrangea arborescens and Hydrangea paniculata. Hydrangea arborescens resembles the bigleaf hydrangea however its blooms are white. This is a lower growing plant that is cut back to the ground each winter. Here is information about a cultivar named "Annabelle" which has bigger blooms than the species. You may need to cut and paste the complete url into your browser ot make it work correctly.
Hydrangea paniculata also has white blooms that last quite a long time, the plant is a woody shrub and requires little pruning. You may know it as PeeGee hydrangea but there are numerous cultivars on the market.
Here are photos of one.
The leaves of the above are not shaped like oak leaves; that plant would be the oak leaf hydrangea or Hydrangea quercifolia. It is not winter hardy in your area.
I hope this helps.
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